A dry etch process may be used in semiconductor wafer processing to remove materials from a surface of a wafer, or from films deposited on a wafer by exposure to plasma. Plasma is an electrically neutral, partially ionized phase of matter. An etch reactor not only produces plasma, but also provides a degree of control of the chemical and physical reactions that occur on the wafer or film surface. Through the etch process, materials are removed from the wafer or film surface in an etching area to form profiles and dimensions that, in part, define circuit elements.
In a known plasma reactor, the plasma is produced in a volume proximate to the wafer and expands to fill most or all of the total reactor chamber volume. The plasma interacts with all of the surfaces the plasma contacts. Outside the proximate wafer volume, the plasma-wall interactions can yield undesirable results such as a sputtering of wall material or more commonly, a deposition on or near the wall. As wall deposits increase in thickness with continued processing, the wall deposits can flake off creating particle contaminants. Additionally, because the wall deposits can have different electrical and chemical properties than the wall itself, the deposits can change how the plasma interacts with the wall and can cause a change in the plasma properties over time. The wall deposits must therefore be periodically removed. In-situ plasma cleaning is preferable, but often difficult or very slow due to the low energy of some plasma-wall interactions. Thus, manual cleaning of the reactor is often required, which increases operational costs and reduces system throughput.
FIG. 1 illustrates a cross-sectional side view of a prior art plasma reactor. The apparatus employs a chamber housing 110 that forms a reactor or chamber 100. Disposed within the top of the housing 110 is a first electrode 112. As shown, the first electrode 112 and the housing 110 are coupled electrically to ground 134. A second electrode 114 is disposed within the lower part of the housing 110, opposite and parallel to the upper electrode 112. The second electrode 114 is electrically isolated from the housing 110 by an insulator ring 116. A substrate or wafer 118 to be etched is placed on an interior face of the second electrode 114, which is often configured with a clamping device and/or a cooling device. The wafer 118 is surrounded by a thin plate 120 fabricated of an insulator material such as quartz.
Etchant gas is supplied to the reactor 100 by an etchant gas supply 122 and a supply line 124. The supply line 124 is connected to the reactor 100 via a port through the first electrode 112 to deliver an etchant gas to the interior of the reactor 100. A reduced pressure is maintained within the reactor 100 by a vacuum pump 128, which is connected to the reactor 100 through a vacuum line 126. Radio Frequency (RF) power is supplied to the second electrode 114 by an RF power supply 130 and an impedance matching network 132.
At the appropriate reduced pressure of etchant gas within the reactor 100 and the application of an appropriate RF power to the second electrode 114, a plasma is formed in the inter-electrode volume 146 between the first electrode 112 and the second electrode 114, and expands to the volume 142 outside the first and the second electrodes 112 and 114. The plasma gas within the volume 142 can interact with exposed interior walls 144 of the chamber housing 110.
Others have attempted to confine plasma proximate to the wafer 118. Some known devices employ two or more annular rings 150 immediately about the inter-electrode volume 146 between two parallel disk electrodes similar to those illustrated in FIG. 2. Added to the reactor 100 of FIG. 1 are multiple annular rings 150 that fill the volume between the upper electrode 112 and lower electrode 114 about their periphery. The annular rings 150 are fabricated from a non-electrically-conductive material, such as quartz, and have small gaps 152 between them. The gaps 152 allow gas to flow from the inter-electrode volume 146 to an outer volume 148, and then to the vacuum pump 128. The gaps 152 are sufficiently narrow and the width of the annular rings 150 sufficiently wide that there is a significant loss of gas flow conductance through the small gaps 152. This gas flow conductance loss creates a pressure differential between the inter-electrode volume 146 and the outer volume 148. The plasma created within the inter-electrode volume 146 is confined to the inter-electrode volume 146 due to the narrow gaps 152 and the very low pressure that exists in the outside volume 148.
The above-described approach to plasma confinement can suffer from a limited processing window. At low plasma operating pressures, generally less than 60 millitorr, the efficacy of the annular rings 150 cannot always establish a beneficial pressure drop. In addition, in instances where the plasma is confined, the low gas flow conductance created by the annular rings 150 limit the gas flow rates that can be employed.
If a plasma can be confined to a volume proximate the wafer, several advantages are gained including enhanced process stability and repeatability, and reduced system maintenance. Accordingly, there is a need for an apparatus and a method that confines the plasma to a volume proximate the wafer while not significantly restricting the pressures and/or gas flow rates of the apparatus and method.